Communication networks of the future will increasingly rely upon optical fibers as the transmission medium. In such a system, an electrical data signal modulates a laser optically coupled to the input end of the fiber. An optical detector at the fiber's output end reconverts the optical signal to an electrical signal. For moderately long distance transmission, the optical wavelength is chosen to fall in the frequency bands around 1.3 and 1.55 .mu.m, which contain the spectral windows for which silica fibers exhibit low optical loss. Heterojunction semiconductor lasers of the InGaAsP material family are under extensive development for such use since they emit in the desired spectral range.
Several types of semiconductor lasers are being considered, one of which is a ridge-waveguide laser 10 illustrated in the side view of FIG. 1. An active layer 12 of n-type InGaAsP is epitaxially formed on a substrate 14 of p-type InP. An n.sup.+ -type layer 16 of InP is epitaxially deposited over the active layer 12, and its upper surface is formed into a ridge 18 extending along an optical axis perpendicular to the illustration. The surface of the n.sup.+ -type layer 16 away from the ridge 18 is covered with an insulating layer 20 of SiO.sub.2. A gold contact layer 22 covers the upper surface but is electrically connected to only the ridge 18. A wafer having multiple replicates of this structure is cleaved at two planes perpendicular to the optical axis so as to form an optical cavity having two facets at the cleaved planes, only one facet 24 being illustrated. Leads 26 and 28 are connected to the gold layer 22 and the substrate 14. A diode is formed at the active layer 12. When the leads 26 and 28 apply a positive voltage to the p-type substrate relative to the n.sup.+ -type layer 16, the diode is forward biased and lasing light is generated in a lasing region 30 in and near the active layer 12 beneath the ridge 18. The opposite polarity of applied voltage is referred to as reverse bias. The ridge 18 also provides index guiding of the lased light along an optical axis extending along the ridge 18 in a direction perpendicular to the illustration and extending through the illustrated facet 24 of the chip. That facet 24 is designed to have a small but finite optical transmission while the unillustrated opposed facet should have as low a transmission as possible. Thereby, lasing is achieved. This type of laser 10 is described only as an example; the following remarks apply to other types of lasers as well.
If these lasers are to be employed in the telephone network, where such lasers need to be fielded in large numbers and in sometimes remote locations, they need to be extremely reliable. Sim et al. provide a general discussion on this topic in "Reliability testing of opto-electronic components," British Telecommunications Technology Journal, volume 4, 1986, pp. 104-113. Semiconductor lasers of the AlGaAs material family, which are used for the 0.8 .mu.m silica transmission window, have been well characterized and are known to be susceptible to formation of dark-line defects and degradation of facets. On the other hand, InGaAsP lasers are less well characterized and are believed to be less susceptible although vulnerability has been predicted. Ueda has reported in "Degradation of III-V Opto-Electronic Devices," Journal of the Electrochemical Society, volume 135, 1988, pp. 11C-21C that a failure mechanism in AlGaAs lasers is caused by defects at the facets 24, which then propagate into the interior of the lasers at high laser power levels. However, he reported that such failure mode does not exist in InGaAsP. He also ascribed the dark-line defect to dislocations and other defects in and adjacent to the active lasing region 30. Again, he failed to detect such defects in InGaAsP. Peek also discusses facet erosion in AlGaAs lasers in "Water Vapor, Facet Erosion, and the Degradation of (al,Ga)As DH Lasers Operated at CW Output Powers of Up to 3 mW/.mu. Stripewidth," Journal of Quantum Electronics, volume QE-17, 1981, pp.781-787. His discussion about InGaAsP is very general, but he predicts less susceptibility.
One of the present inventors together with others originated a sulfide passivation technique for bipolar transistors of GaAs and other III-V compounds, as disclosed in U.S. Pat. Nos. 4,751,200 and 4,751,201. Kawanishi et al. have applied this sulfide passivation to the facets of an AlGaAs semiconductor laser, as disclosed in "Effects of (NH.sub.4).sub.2 S Treatments on the Characteristics of AlGaAs Laser Diodes," Proceedings of the 21st Conference on Solid State Devices and Materials, Tokyo, Japan, 1989, pp. 337-340. The passivation allows higher drive currents without catastrophic failure. The passivation was motivated by a desire to reduce facet degradation. The usefulness of these results in InGaAsP is cast into doubt by the statement of Nottenburg et al. that the surface recombination of InGaAs is orders of magnitude less than that of GaAs. See "Hot-electron InGaAs/InP heterostructure bipolar transistors with f.sub.r of 110 GHz," IEEE Electron Device Letters, volume 10, 1989, pp.30-32.